What is the most accurate pretrained sentiment analysis model by 2019?
For best results, I'd recommend Google Cloud Machine Learning. It has [Natural Language Processing API] (https://cloud.google.com/natural-language/docs/basics) with Sentiment, Entity, and Entity-Sentiment analysis.

You can implement in C++, PHP, Python, or other languages. This does require running a virtual machine instance on Google Cloud. TensorFlow can also be used for NLP sentiment analysis. The only drawback is it's not economical.

As an alternative, this is something I came across recently by Fast Text : http://fasttext.cc.
They have some pre-trained models trained on yelp and some other datasets. They support a lot of languages